Fix deprecated keyWindow usage in FIAM

[paulb777]

Description

FirebaseInAppMessaging/Sources/DefaultUI/Banner/FIRIAMBannerViewController.m:166:43: warning: 'keyWindow' is deprecated: first deprecated in iOS 13.0 - Should not be used for applications that support multiple scenes as it returns a key window across all connected scenes [-Wdeprecated-declarations]

Reproducing the issue

Build in release-11.0 branch

Firebase SDK Version

11.0

Xcode Version

15.4

Installation Method

CocoaPods

Firebase Product(s)

In-App Messaging

Targeted Platforms

iOS

Relevant Log Output

No response

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